Incorporating Structure Evolution History in Uncertainty Analysis for Extremely Heterogeneous Reservoir-A Case from a Carbonate Reservoir in Middle East

The facies model is the foundation of the geological model and is crucial for reserve estimation and development optimization. For some reservoirs, the geological model faces significant uncertainty due to the limited number and available data. This paper takes a highly heterogeneous carbonate reservoir in the Middle East as an example, by incorporating the structural evolution into the uncertainty analysis, clarifies uncertainties in facies distribution and provides guidance for oilfield development. This paper proposes a methodology for studying facies models and reserve uncertainties by integrating structural evolution. Firstly, sedimentary, and diagenetic data are analyzed to identify the main controlling factors of reservoir properties. Then, based on regional geological background and seismic interpretation, the structural evolution history of the reservoir is clarified. Subsequently, by integrating structural evolution insights and current reservoir understanding, low/base/high case facies distribution maps are quantitatively constructed. These maps serve as the basis for facies modeling and reserve uncertainty analysis. The study identifies the impact of structural evolution-driven facies model uncertainties and proposes countermeasures to reduce these uncertainties. The study indicates that facies model uncertainty primarily arises from the facies type planar distribution which was controlled by structural evolution. The main reservoir is inner ramp deposit, with cementation as the primary controlling factor of reservoir properties. The Triassic structure of the oilfield tilts southwestward, with the southwestern part intruding into water, experiencing intense diagenesis and resulting in poor reservoir properties. Conversely, cementation in the northeastern direction is weaker, leading to better reservoir properties. Currently, drilling activities are concentrated in the south and central part of the oilfield, with no new wells in the northern part. This makes it challenging to delineate the distribution of high-quality reservoirs in the northeastern direction. Using the relationship between structural tilting and reservoir properties, low-case, base-case, and high-case facies distribution maps were constructed, sensitivity and uncertainty analyses were carried out. Results show that this factor has the most significant impact, influencing up to 40% of the base-case reserves. Additionally, the variogram parameters of the facies model also exert some influence, with horizontal range being more impactful than vertical range, while the variogram's direction has a smaller effect. Based on these insights, exploration wells are recommended in the northeastern part to investigate the distribution range of high-quality reservoirs. For oilfields in the early development stage or those with smaller scales, limited investments and data often lead to significant uncertainties. This study examines the relationship between structural evolution and reservoir properties, incorporating it into a systematic uncertainty analysis workflow. The primary sources of geological reserve uncertainty are identified, and directions for further research to reduce uncertainty in oilfield development are proposed.